Synapse formation, synapse specificity and neurotransmission were studied using neurons and muscle cells in culture. Synapses were detected and investigated by electrophysiological recording. Neurons from chick and rat retina and chick spinal cord form synapses with muscle cells. Each type of neuron possesses a synapse-competent state in its development during which it can abundantly form synapses with muscle cells in culture. In addition, developmental periods for maximal neurite extension correlate with the developmental periods for synaptogenesis. These results suggest a mechanism of synapse specificity based on a synaptogenic period during development for each type of neuron.